Heterojunction bipolar transistor containing at least one silicon carbide layer

ABSTRACT

A bipolar transistor includes a collector that is selected from the group SiC and SiC polytypes (4H, 6H, 15R, 3C . . . ), a base that is selected from the group Si, Ge and SiGe, at least a first emitter that is selected from the group Si, SiGe, SiC, amorphous-Si, amorphous-SiC and diamond-like carbon, and at least a second emitter that is selected from the group Si, SiGe, SiC, amorphous-Si, amorphous-SiC and diamond-like carbon. Direct-wafer-bonding is used to assemble the bipolar transistor. In an embodiment the bandgap of the collector, the bandgap of the at least a first emitter and the bandgap of the at least a second emitter are larger than the bandgap of the base.

RELATED PATENT APPLICATIONS

This application claims the priority of U.S. Provisional PatentApplication Ser. No. 60/333,258, filed on Nov. 21, 2001, entitledBIPOLAR TRANSISTOR CONTAINING AT LEAST ONE SILICON CARBIDE LAYER,assigned to Astralux, Inc., incorporated herein by reference.

Non-provisional U.S. patent application Ser. No. 10/273,041, filed Oct.10, 2002, entitled DOUBLE HETEROJUNCTION LIGHT EMITTING DIODES AND LASERDIODES HAVING QUANTUM DOT SILICON CARBIDE EMITTERS, assigned toAstralux, Inc., incorporated herein by reference, provides for thefabrication of silicon-based light emitting diodes using nano-patterningand direct-wafer-bonding.

FIELD OF THE INVENTION

This invention relates to the field of active solid-state devices, andmore specifically to bipolar transistors, also called semiconductortriodes, that include silicon carbide (SiC).

BACKGROUND OF THE INVENTION

Bipolar junction transistors (BJTs) are thought to have been invented inabout 1948, for example see U.S. Pat. Nos. 2,524,033 and 2,569,347,incorporated herein by reference, wherein the use of semiconductors suchas SiC was mentioned. In addition U.S. Pat. Nos. 2,918,396, 4,945,394,5,610,411 and 6,329,675, incorporated herein by reference, are examplesof patents that use SiC in BJTs.

A number of different semiconductor-device structures have beenpublished and/or commercially developed. Heterojunction bipolartransistors (HBTs) (see above-mentioned U.S. Pat. No. 2,569,347) weremade using a variety of different semiconductors in the same devicestructure. For example U.S. Pat. No. 4,985,742 by J. Pankove describes aGaN/SiC HBT, incorporated herein by reference.

In accordance with the present invention, direct-wafer-bonding (forexample see Appl. Phys. Lett. 56, p.737, 1990, by Z. L. Liau and D. E.Mull) provides an elegant and cost effective alternative for formingwide-bandgap heterojunctions, wherein direct-wafer-bonding is performedusing commercially available wafers and standard device processing.

Wafer-bonding of dissimilar semiconductors is a technology that hasfacilitated the manufacture of red AlInGaP/GaP light emitting diodes(LEDs) (see Appl. Phys. Lett. 56, p. 737, 1990, by Z. L. Liau and D. E.Mull), mirror stacks for long-wavelength VCSELs (see IEEE Photon.Technol. Lett. 7, 1225, 1995, by D. I. Babic et al), and Si/InGaAs p-i-nphotodetectors (see Appl. Phys. Lett. 70, 2449, 1997, by B. F. Levine etal) with near-perfect interfaces, wherein the above-mentioned AlInGaPLEDs are manufactured using a high-volume production process, resultingin low cost products.

SUMMARY OF THE INVENTION

This invention provides new and unusual HBT structures that contain atleast one SiC layer.

In accordance with a feature of this invention, the fabrication of HBTshaving one or more heterojunctions utilizes a direct-wafer-bondingprocess by combining semiconductor materials that have incompatiblegrowth technologies.

Active solid-state devices in accordance with the present invention findutility in a number of fields, including, but not limited to, RF poweramplifiers used in wireless communication and radar applications, andpower switches that are needed for traction control in electricvehicles. Direct-wafer-bonded bipolar transistors in accordance withthis invention can be used as RF power devices and near-DC powerswitches. Direct-wafer-bonded heterojunctions in accordance with theinvention can also be used in thyristors and heterojunctionfield-effect-transistors (HFETs).

SiC is a wide-bandgap semiconductor material that has extraordinaryproperties, for example high thermal conductivity, high breakdown field,and high saturated electron velocity. SiC can be made with either n-typeconductivity or p-type conductivity, and various p-n diodes and n-p-nBJTs have been demonstrated. (For example see (1) Tang Y., Fedison J.B., and Chow T. P., An Implanted-Emitter 4H—SiC Bipolar Transistor withHigh Current Gain, El. Dev. Lett., Vol. 22, No. 3, pp119-120, 2001; and(2) Ryu S. H., Agarwal A. K., Singh R., and Palmour J., 1800V NPNBipolar Junction Transistors in 4H—SiC, El. Dev. Lett., Vol. 22, No. 3,pp 124-126, 2001.)

However, SiC-based HBTs are not known to have been demonstrated prior tothe present invention due to problems such as SiC bandgap engineeringrelated to the binary nature of SiC, and due to the difficulty ofgrowing different SiC polytypes together.

This invention provides discrete SiC bandgap engineering using adirect-wafer-bonding process, which process achieves the combination ofSiC and Si for bipolar solid-state device applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a SiC-based HBT in accordance withthe invention.

FIG. 2 is a cross section of a SiC-based double heterojunction bipolartransistor (DHBT) in accordance with the invention wherein the bandgapof the collector and the bandgap of the two emitters are larger than thebandgap of the base.

DETAILED DESCRIPTION

A bipolar transistor includes three distinct semiconductor regions,respectively called an emitter region, a base region and a collectorregion. A cross-sectional schematic of a two-finger SiC-based HBT 10 inaccordance with the present invention is shown in FIG. 1. Moregenerically, HBT 10 (which has two emitter elements and three baseelements) has one or more emitter elements and one or more baseelements.

HBT 10 includes (1) a first Si n-type emitter layer 11 and its contactlayer 12, (2). a second Si n-type emitter layer 13 and its contact layer14, (3) a p-type Si base layer 15 and its three contact layers 16, 17,and 18, (4) an n-type SiC collector layer 19, and (5) a n-type SiCsub-collector layer 20 having a contact layer 21, wherein sub-collectorlayer 20 may be more heavily doped than collector layer 19, thusimproving the ohmic contact to the collector layer 19.

In the above arrangement, the contact layers to the Si layers may beeither poly-Si, contact metals such as Aluminum or tungsten, or a metalsilicide.

In operation, HBT 10 is a three-terminal device wherein contact layer 21provides the device's collector-terminal, the three contact layers 16,17 and 18 are electrically connected together to form the device'sbase-terminal, and the two contact layers 12 and 14 are electricallyconnected together to form the device's emitter-terminal. In the case ofan N-finger HBT 10, the number N of emitter contact layers areelectrically connected together to form the device's emitter-terminal,and the base contact layers are electrically connected together to formthe device's base-terminal.

HBT 10 includes a first interface 31 between emitters 11/13 and base 15,a second interface 22 between base 15 and collector 19, and a thirdinterface 23 between collector 19 and sub-collector 20. In accordancewith a feature of the invention, at least one, two, or all three of theinterfaces 31, 22 and 23 is a direct-wafer-bonded interface.

For example, emitter-to-base interface 31 may be either a growninterface, an implanted or a diffused interface, whereas bothbase-to-collector interface 22 and collector-to-sub-collector interface23 may be direct-wafer-bonded-interfaces.

Within the spirit and scope of this invention there are many variationsand combinations of techniques well known to semiconductor devicespecialists that can be applied in order to fabricate devices asabove-described.

As the term is used herein, direct-wafer-bonding is intended to mean aprocess whereby two smooth and flat surfaces are brought together, inphysical contact, in the absence of an intermediate layer or film, andusually with the application of a uniaxial pressure, such that the twoflat surfaces are locally attracted to each other by Van der Wallsforces, so that the two flat surfaces stick or bond together. Thecrystallites in the two flat surfaces of a direct-wafer-bonded interfacecan fuse together at elevated temperatures due to thesurface-energy-induced migration and crystal growth, or the formation ofbonds, between the two surface species.

Prior to the direct-wafer-bonding, the two surfaces that are to bebonded are processed to produce surface characteristics that facilitatethe direct-wafer-bonding of these two surfaces. The two mating surfacesare prepared for direct-wafer-bonding, as is well known to those skilledin the art. Generally, the two surfaces must be clean, they must beflat, and these two surfaces do not contain an intermediate materialsuch as an oxide, such that direct-wafer-bonding can be achieved.Direct-wafer-bonding without the presence of an intermediate oxide layeralso facilitates electrical conduction across the bonded interface.

HBT 10 can be made to provide n-p-n conductivity wherein emitters 11 and13 are n-type Si, wherein base 15 is p-type Si, and wherein collector 19and sub-collector 20 are n-type SiC, or HBT 10 can be made to providep-n-p conductivity wherein emitters 11 and 13 are p-type Si, whereinbase 15 is n-type Si, and wherein collector 19 and sub-collector 20 arep-type SiC.

The n-emitter/p-base/n-collector structure is preferred as thisstructure expected to have superior transport properties, due to ahigher electron-mobility than hole-mobility in Si and SiC.

In an embodiment of the invention HBT 10 provided a base/collectordirect-wafer-bonded heterojunction 22, thus differing from aconventional HBT which employs a grown emitter/base heterojunction.

Using Si within emitter regions 11 and 13 and within base region 15 ofHBT 10, and using SiC in the collector region 19 and sub-collectorregion 20 of HBT 10, allows under normal operating conditions that ahigh electrical field resides within the SiC collector due to the SiC'shigh maximum electric field before breakdown. Efficient heat removalfrom SiC-based HBT 10 is assured due to the high thermal conductivity ofthe SiC.

Si base 15 of HBT 10 facilitates good base electron-transport due to thehigh electron-mobility that is provided by Si. Using Si as the baselayer also results in a low base sheet resistance and a low resistivityohmic p-type contact that base 22 makes with the contacts 16, 17 and 18.The critical p-type base layer should be doped above 10¹⁷ cm⁻³ and thethickness should be less than 1 micrometer.

The emitter/base portion of HBT 10 can be fabricated using standardphotolithography and etching (reactive ion etching and wet/or chemicaletching). Passivation or protection of HBT 10 can be achieved usingsilicon oxide or silicon nitride, as is shown at 30.

Other HBT structures can be made by changing emitters 11/13 to amaterial that has a larger bandgap energy than that of the Si base layer15, for example, but not limited to using SiC, amorphous-Si,amorphous-SiC, or diamond-like carbon for emitters 11,13. Such an HBTstructure with two heterojunctions is sometimes called a doubleheterojunction bipolar transistor (DHBT), shown in FIG. 2.

FIG. 2 is a cross section of a SiC-based DHBT 50 in accordance with theinvention wherein the bandgap E_(g,C) of collector layer 51 and thebandgap E_(g,E) of emitter layer 52 is larger that the bandgap E_(g,B)of base layer 53. It is also advantageous that the emitter-baseheterojunction forms a Type I heterojunction as defined by Weisbuch andVinter (Quantum Semiconductor Structures, Academic Press, London 1991,p. 3) to maximize the emitter injection efficiency.

The emitter 52, the base 53, the collector 51 and the sub-collector 20of the FIG. 2 device can be constructed using the materials describedrelative to the FIG. 1 device.

Advantages of using DHBT 50, as opposed to using the HBT 10 structureabove-described relative to FIG. 1, includes increased injectionefficiency and increased gain of DHBT 50, while simultaneouslymaintaining good base-transport, low sheet resistance and good p-typebase contact, all while keeping the high field within the SiC driftregion.

When diamond-like carbon is used in emitters 11/13 of FIG. 1 theresulting heterojunction structure must be p-n-p, until such time asn-type diamond becomes available.

Emitters 11/13 of the FIG. 1 structure can be direct-wafer-bonded, grownor deposited prior to or after direct-wafer-bonding of the Si base 15 tothe SiC collector 19. That is, interfaces 31 can also bedirect-wafer-bonded interfaces.

FIG. 2's DHBT 50 can also involve using a SiGe alloy to form thebase/emitter layers of an emitter/base heterojunction combination thatis direct-wafer-bonded to SiC collector 51. The advantage of this FIG. 2structure is similar to that above-described. SiGe is a binarysemiconductor in which the bandgap depends upon the alloy composition.

In accordance with this invention the SiC collector can be increased inthickness beyond that which is possible by growth bydirect-wafer-bonding two or more grown collector layers together to forma multi-layer collector.

Direct-wafer-bonding using both polarities of SiC (i.e. Si—Si and Si—C),and using Si wafers cut on or 3.5 degrees or 8 degrees off the (0001)axis is within the spirit and scope of this invention. Various SiCpolytypes (4H, 6H, 3C, 15R . . . ) can also be used. The Sic should beof either (100) or (111) crystal orientation.

Surface morphology and surface preparation procedures at theabove-described direct-wafer-bonded interfaces are important. It hasbeen identified that the root-mean-squared surface roughness of such aninterface should be better than about 10 Angstroms, as measured byatomic force microscopy. Such a surface roughness can be provided bypolishing and in-situ hydrogen etching.

Growing a sacrificial silicon oxide (SiO₂) layer on a surface that is tobe direct-wafer-bonded, and subsequently etching the silicon oxide layeroff in hydrofluoric acid, improves the surface's morphology and protectsthe surface immediately prior to direct-wafer-bonding.

An issue to be considered when direct-wafer-bonding SiC to Si forvertical device structures is that both materials readily oxidize inair. This oxide needs to be removed. Thus it is preferred that thedirect-wafer-bonding and any subsequent annealing step take place in aninert or a reducing atmosphere.

Standard preparation of surfaces to be direct-wafer-bonded includes theuse of sacrificial oxides followed by solvent, RCA (for example see W.Kern, D. A. Puotinen, RCA Review, p. 187, June 1970), and electronicgrade hydrofluoric acid cleaning.

Both hydrophilic and hydrophobic surfaces were used whendirect-wafer-bonding as above-described.

Direct-wafer-bonding as above-described was performed using anall-graphite wafer bonder that provided chemical stability, uniformthermal expansion at high temperatures, and using a process wherebyknown and calibrated uniaxial pressures were applied.

In a non-limiting example of the invention, direct-wafer-bonding wasaccomplished by applying a pressure of up to about 600 psi and annealedfor up to about 60 in at between 500 degrees C. and 1000 degrees C. inboth inert (nitrogen and argon) and reducing (forming gases) atmospheresin order to solidify the direct-wafer-bond. When fabricating Ts using Siand SiGe it is important that the annealing temperature stay below 700degrees C. to minimize dopant diffusion. Annealing at highertemperatures might be possible using rapid the at processing by reducingthe annealing duration.

The following two heterojunction p-n diodes (1) a Si base and a 4H—SiCcollector and (2) a Si base and a 6H—SiC collector, both in accordancewith this invention, have been demonstrated and exhibit excellentcurrent-voltage characteristics. (100) Si was bonded to Si-face, C-face,on-axis (4H and 6H), 3.5 degrees off (0001) (6H) and 8 degrees off(0001) (4H) SiC.

The invention has been described in detail while making reference toembodiments thereof However, since it is known that others, uponlearning of this invention, will readily visualize yet other embodimentsthat are within the spirit and scope of this invention, this detaileddescription is not to be taken as a limitation on the spirit and scopeof the invention.

1. A bipolar transistor comprising: a collector selected from one ormore of the group SiC, (4H, 6H, 15R, 3C . . . ); a base selected fromone or more of the group Si, Ge and SiGe; an emitter selected from oneor more of the group Si, SiGe, SiC, amorphous-Si, amorphous-SiC anddiamond-like carbon; and a sub-collector selected from one or more ofthe group SiC, (4H, 6H, 15R, 3C . . . ).
 2. The bipolar transistor ofclaim 1 wherein at east one of a first interface between said collectorand said base and a second interface between said base and said emitteris a direct-wafer-bonded interface.
 3. The bipolar transistor of claim 1wherein at least one of a first interface between said collector andsaid base, a second interface between said base and said emitter, and athird interface between said collector and said sub-collector is adirect-wafer-bonded interface.
 4. The bipolar transistor of claim 1wherein said emitter is Si, said base is Si, and said collector is SiC.5. The bipolar transistor of claim 4 wherein at least one of a firstinterface between said SiC collector and said Si base and a secondinterface between said Si base and said Si emitter is adirect-wafer-bonded interface.
 6. The bipolar transistor of claim 1wherein said sub-collector is SiC, said collector is SiC, and said baseis Si and wherein at least one of a first interface between said SiCsub-collector and said SiC collector, a second interface between saidSiC collector and said Si base, and a third interface between said Sibase and said Si emitter is a direct-wafer-bonded interface.
 7. Thebipolar transistor of claim 1 wherein said emitter is Si, said base isGe, and said collector is SiC.
 8. The bipolar transistor of claim 7wherein at least one of a first interface between said SiC collector andsaid Ge base and a second interface between said Ge base and said Siemitter is a direct-wafer-bonded interface.
 9. The bipolar transistor ofclaim 1 wherein bandgap of said collector and a bandgap of said emitterare larger than a bandgap of said base.
 10. The bipolar transistor ofclaim 9 wherein at least one of a first interface between said collectorand said base and a second interface between said base and said emitteris a direct-wafer-bonded interface.
 11. The bipolar transistor of claim10 further including: a sub-collector selected from one or more of thegroup SiC and SiC polytypes (4H, 6H, 3C, 15R . . . ).
 12. The bipolartransistor of claim 11 wherein at least one of a first interface betweensaid collector and said base, a second interface between said base andsaid emitter, and a third interface between said collector and saidsub-collector are a direct-wafer-bonded interface.
 13. The bipolartransistor of claim 1 wherein said emitter is SiGe, said base is SiGe,and said collector is SiC.
 14. The bipolar transistor of claim 13wherein bandgap of said emitter is greater than a bandgap of said base.15. A bipolar transistor comprising: a collector selected from the groupSiC and SiC polytypes (4H, 6H, 15R, 3C . . . ), said collector having acollector-surface; a base selected from the group Si, Ge and SiGe, saidbase having first base-surface engaging said collector-surface, and saidbase having a second base-surface; at least a first emitter selectedfrom the group Si, SiGe, SiC, amorphous-Si, amorphous-SiC anddiamond-like carbon engaging said second base-surface: and at least asecond emitter selected from the group Si, SiGe, SiC, amorphous-Si,amorphous-SiC and diamond-like carbon, said second emitter being spacefrom said at least a first emitter, and said at least a second emitterengaging said second base-surface.
 16. The bipolar transistor of claim15 wherein at least one of a first interface between saidcollector-surface and said first base-surface, second interface betweensaid second base-surface and said at least a first emitter, an a thirdinterface between said second base-surface and said at least a secondemitter is a direct-wafer-bonded interface.
 17. The bipolar transistorof claim 15 wherein a bandgap of said collector, a bandgap of said atleast a first emitter and a bandgap of said at least a second emitterare larger than a bandgap of said base.
 18. The bipolar transistor ofclaim 17 wherein at least one of a first interface between saidcollector-surface and said first base-surface, a second interfacebetween said second base-surface and said at least a first emitter, an athird interface between said second base-surface and said at least asecond emitter is a direct-wafer-bonded interface.
 19. A heterojunctioncomprising: a first semiconductor layer selected from the group Si andSi_(l)Ge_(l-x); said first semiconductor layer having a top surface anda bottom surface; a first SiC layer; said first SiC layer having a topsurface and a bottom surface; a direct-wafer-bonded interface betweensaid bottom surface of said first semiconductor layer and said topsurface of said SiC layer; a Si layer; said Si layer having a topsurface and a bottom surface; a direct-wafer-bonded interface betweensaid top surface of said first semiconductor layer and said bottomsurface of said Si layer; a second SiC layer; said second SiC layerhaving a top surface and a bottom surface; and a direct-wafer-bondedinterface between said bottom surface of said first SiC layer and saidtop surface of said second SiC layer.
 20. A bipolar transistorcomprising: a collector selected from one or more of the group SiC, (4H,6H, 15R, 3C . . . ); a base selected from one or more of the group Si,Ge and SiGe; an emitter selected from one or more of the group Si, SiGe,SiC, amorphous-Si, amorphous-SiC and diamond-like carbon; and a SiCsub-collector.
 21. The bipolar transistor of claim 20 wherein thecollector is SiC, the base is Si, and the emitter is Si and wherein atleast one of a first interface between said SiC sub-collector and saidSiC collector, a second interface between said SiC collector and said Sibase, and a third interface between said Si base and said Si emitter isa direct-wafer-bonded interface.
 22. A bipolar transistor comprising: aSiC collector; a Ge base; and an Si emitter.
 23. The bipolar transistorof claim 22 wherein at least one of a first interface between said SiCcollector and said Ge base and a second interface between said Ge baseand said Si emitter is a direct-wafer-bonded interface.
 24. The bipolartransistor of claim 23 further including: a SiC sub-collector.
 25. Abipolar transistor comprising: a SiC collector; a base selected from oneor more of the group Ge and SiGe; and an emitter selected from one ormore of the group Si, SiGe, SiC, amorphous-Si, amorphous-SiC, anddiamond-like carbon.
 26. The bipolar transistor of claim 25 wherein atleast one of a first interface between said SiC collector and said baseand a second interface between said base and said emitter is adirect-wafer-bonded interface and wherein the base comprises Si.
 27. Thebipolar transistor of claim 25 further including: a SiC sub-collector.28. The bipolar transistor of claim 27 wherein at least one of a firstinterface between said SiC sub-collector and said SiC collector, asecond interface between said SiC collector and said base, and a thirdinterface between said base and said emitter is a direct-wafer-bondedinterface.
 29. The bipolar transistor of claim 25 wherein the basecomprises Ge.
 30. A bipolar transistor comprising: a SiC collector; abase selected from one or more of the group Si, Ge and SiGe; and anemitter selected from one or more of the group SiGe, amorphous-Si,amorphous-SiC, and diamond-like carbon.
 31. The bipolar transistor ofclaim 30 wherein at least one of a first interface between said SiCcollector and said base and a second interface between said base andsaid emitter is a direct-wafer-bonded interface and wherein the basecomprises Si.
 32. The bipolar transistor of claim 30 further including:a SiC sub-collector.
 33. The bipolar transistor of claim 32 wherein atleast one of a first interface between said SiC sub-collector and saidSiC collector, a second interface between said SiC collector and saidbase, and a third interface between said base and said emitter is adirect-wafer-bonded interface.
 34. The bipolar transistor of claim 30wherein the bas comprises Ge.
 35. The bipolar transistor of claim 30wherein the emitter comprises Si.
 36. The bipolar transistor of claim 30wherein the emitter is SiGe.
 37. The bipolar transistor of claim 30wherein the emitter comprises carbon.
 38. The bipolar transistor ofclaim 30 wherein the base is positioned between the emitter and thecollector.
 39. The bipolar transistor of claim 30 wherein the emitter isamorphous-SiC.
 40. The bipolar transistor of claim 30 wherein theemitter is diamond-like carbon.
 41. A bipolar transistor comprising: acollector selected from one or more of the group SiC, (4H, 6H, 15R, 3C .. . ); a base selected from one or more of the group Si, Ge and SiGe; anemitter selected from one or more of the group Si, SiGe, SiC,amorphous-Si, amorphous-SiC and diamond-like carbon, wherein a bandgapof said collector and a bandgap of said emitter are larger than abandgap of said base and wherein at least one of a first interfacebetween said collector and said base and a second interface between saidbase and said emitter is a direct-wafer-bonded interface; and asub-collector selected from one or more of the group SiC and SiCpolytypes (4H, 6H, 3C, 15R . . . ).
 42. The bipolar transistor of claim41 wherein at least one of a first interface between said collector andsaid base, a second interface between said base and said emitter, and athird interface between said collector and said sub-collector are adirect-wafer-bonded interface.
 43. A bipolar transistor comprising: aSiC collector; an SiGe base; and an SiGe emitter.
 44. The bipolartransistor of claim 43 wherein a bandgap of said emitter is greater thana bandgap of said base.